Polypropylene-based Thermoplastic Olefin Compositions (TPO) are blends of polypropylene, an elastomer, optional propylene-based elastomers, and optional non-polymeric components such as fillers and other compounding ingredients. Typically, TPOs are multiphase polymer blends where a polypropylene forms a continuous matrix phase and the elastomer component, generally derived from an ethylene containing interpolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO, while the ethylene polymer imparts flexibility and impact resistance. Typically, TPOs have a dispersed phase which is not, or only modestly, cross-linked.
It has been observed that certain non-crosslinked TPO compositions have improved processability, and also improved mechanical properties, when the compositions contain high levels of isotactic polypropylene, for example, above 70 wt %. Generally, one of the problems with adding more isotactic polypropylene to any thermoplastic composition is a noticeable drop in ductility. For example, any improvement in a mechanical or impact property, such as Notched Izod, tends to be accompanied by an undesirably high loss of flexibility, for example, flexural modulus. Such a trade-off is of great concern for the makers of automotive parts, particularly car bumpers. Certain compositions described above provide a combination of (a) high Notched Izod together with (b) high flexural modulus. In addition to having superior mechanical properties, the compositions also have desirable molding properties, for example, sufficiently high MFR, without losing, their superior impact strength and flexibility.
Traditionally, very low density ethylene-propylene copolymers and ethylene-propylene-diene terpolymers have been used as the modifier component in TPO compositions, these EP or EPDMs generally have a high molecular weight expressed in Mooney units. Recently, other ethylene-alpha olefin copolymers have been used, especially very low density ethylene-butene, ethylene-hexene and ethylene-octene copolymers which generally have a lower molecular weight expressed in Melt Index units. The density of these latter polymers is generally less than 0.900 g/cm3, indicative of some residual crystallinity in the polymer. The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components such as door skin, air bag cover, side pillars and the like. These parts are generally made using an injection molding process. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate greater than 35 g/10 min). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength.
There is a need therefore for TPO manufacturers to be able to broaden the scope of polymers available to manufacture end use items with a better balance between the performance of the hetero phase composition in its end use, the processability during conversion of the molten compositions into the end use article and the cost of providing those properties.
References of interest include U.S. Pat. Nos. 6,245,856; 6,288,171; 6,245,856; 6,232,402; 5,959,030; US 2009/053959; WO 97/20888; U.S. Ser. No. 14/325,449 filed Jul. 8, 2014, and EP 0 792 914.